1. Field of the Invention
The present invention relates to gradation conversion calibration methods, and gradation conversion calibration modules using the same, for determining a correction amount to be used when converting the image data gradation to the print data gradation in image forming devices that print color images by forming them on recording media.
2. Description of the Related Art
The use of color charts made of test images for each of the three basic colors that are used to create a color image to perform gradation conversion calibration processing (also called uniformity correction processing) in order to determine the correction amount to use when converting the image data gradation to the print data gradation suited for the printer being used is known (for example, JP 2000-33732A (paragraph numbers 0062 through 0072, FIG. 12 and FIG. 13)). With the gradation conversion calibration processing according to Patent Document 1, first an image recording device prints a test chart image for calibration in which patches of the three basic colors, such as C (cyan), M (magenta), and Y (yellow), have been recorded, then the density of each patch of the test chart image is measured using a density measurement device, and then, based on the measured density values and the target density values to be reproduced, the image data conversion conditions under which the input image data are converted to an output image signal (print data) are determined in such a manner that proper image recording can be performed so that a desired image density can be reproduced from the input image data.
The use of a grayscale chart made of grayscale test images that are obtained by combining each color of the three basic colors to perform gradation conversion calibration processing to determine the correction amount to use when converting the image data gradation to that for print data that are suited for the printer being used also is known (for example, JP 2003-94732A (paragraph numbers 0033-0034, FIGS. 5 and 6).
Since there are different advantages and disadvantages to gradation conversion calibration employing a color chart and gradation conversion calibration employing a grayscale chart, there are also known gradation conversion calibration processing methods in which both a color chart and a grayscale chart are used (for example, JP 2003-274199A (paragraph numbers 0020 to 0027, FIGS. 3 and 4)). In this gradation conversion calibration, a test print furnished with single color patches in 21 steps for each color C, M, Y, and K, and grayscale patches in 13 steps, is printed, and the operator for example sets parameters in the I/O relationship of the grayscale patches and the I/O relationship of the single color patches that determine whether or not to give priority to the grayscale balance or give priority to the hue of the individual colors, so as to determine the gradation correction data, and the gradation correction data that are determined are suitably interpolated so that they may be stored on a gradation correction curve storage portion as a gradation correction curve.
In addition to these, there also is known a technology in which the gradation conversion table that is obtained using the color chart and the gradation conversion table that is obtained using the grayscale chart are utilized at a ratio that is appropriate for the image data while using weighting coefficients (for example, JP 2003-116017A, (paragraph numbers 0008 to 0012, FIG. 8)).
When calibration is performed using the color chart in the case of gradation conversion calibration on a dot laser printer, for example, in which a plurality of optical elements that are arranged in a line array are controlled according to the exposure gradation value based on the print data to expose a light-sensitive material, initially it is possible to obtain the result of a quick convergence, however, this convergence value is relatively inaccurate, and the final image quality that is obtained tends to be worse than when calibration is performed using a grayscale chart. This is thought to occur due to shifting in the registration (misregistration) and improper coloring (where changes in specific colors affect other colors as well). However, performing calibration using a grayscale chart has the problem that the number of repeats that are performed before convergence is achieved is large. In addition to this, providing both a gradation conversion table that is obtained using the color chart and a gradation conversion table that is obtained using the grayscale chart is disadvantageous in terms of cost and in terms of the time required for the calibration task.